The nuclear pore complex (NPC) is a large assembly 1200-1400 angstroms in diameter and 600-800 angstroms thick which spans the nuclear envelope. The NPC is the communication pathway between the nucleus and the cytoplasm, allowing the passive diffusion of small molecules and ions and actively transporting large macromolecules and ribonucleoprotein particles between these two compartments. Adeno and herpes viruses bind to the NPC prior to release of their DNA into the nucleus. Intact retrovirus capsids are transported through the NPC. Despite the NPC's prominent location in the cell and its importance in controlling nucleoplasmic exchange of (e.g.) hormone receptor complexes, regulatory proteins, mRNPs, ribosomes and virus components, there is relatively little known about this assembly: the NPC cannot be isolated in sufficient quantities for biochemical characterization or analysis by x-ray methods, only very low resolution 3-D information is available on its structure vary few (<10%) NPC proteins have been identified and the molecular mechanisms of the transport processes are unknown. The long term goals of the work are to understand the structure of the NPC ar high resolution and to elucidate the molecular mechanism of active nucleocytoplasmic transport. The accomplishment of the work described herein will represent major progress towards attaining these goals. Cryo-electron microscopy coupled with computer image processing will be used to determine projection maps and a 3-dimensional map at a resolution of about 50 Angstroms from improved preparations of detergent-released NPCs. Subsequently, the 3-D location of a number of important NPC proteins-- including those necessary for nuclear transport -- will be determined by immunoelectron microscopy and image analysis. An examination of freeze-dried, metal-shadowed preparations of manually spread nuclear envelopes will reveal structural differences on the nuclear and cytoplasmic sides of the NPC. These findings will be related to details in the 3-D map. Experiments using synthetic nuclear localization signal peptides have been designed to investigate the identity of the central plug of the NPC and may provide structural information on the transport apparatus.